It is useful and desirable to develop assurance of the physical proximity of the user of a device, such as a multifunction device. The user may employ a mobile device to control the multifunction device using a standard protocol such as TCP/IP, but the communication protocol does not itself provide any assurance of physical proximity. For example, a TCP/IP connection provides no assurances that the user is not in another country, nor outside the building in the parking lot.
Assurance of physical proximity allows well-understood and easy-to-use concepts of physical security to be leveraged (e.g., locked doors, required passes, guards at entrances, etc.). Existing methods for detecting proximity include those that require typing, which is inconvenient using many mobile devices.